Radio communication system



July 5, 1949. G. G. BRUCK ET A1.. 2,475,474

RADIO COMMUNICATION SYSTEM Filed Feb. 27, 1946 v 2 sheets-sheet 1 Qd a v /A/vfwz-Ms G50/fsf G. a/i'z/c/r Pff/UP E. voLz Fm. Pam-sco@ Va Mm. cam c. vosawre/-f July 5, 1949 G. G. BRUCK ETAL RADIO COMMUNICATION SYSTEM 2 Sheets-Sheet 2 Filed Feb. 27, 194e A s.A M. wm Nm Mm om W IG@ All T. mmllu \WNB005 .MGVCW/ wm .GPNC @l0 OLPM 5M .L GPJO V.. um mM VAE 1--- .MM SIMM u@ vm Patentecl July 5, 1949 UNITED STATES PATENT OFFICE RADIO COMMUNICATION SYSTEM tion of Delaware Application February 27, 1946, Serial No. 650,716

(Cl. Z50-6) 4 Claims.

This invention relates to radio communication systems, and more particularly to a terminal station for a frequency-modulated, radio communication system comprising, for example, two widely separated terminal stations and a plurality of intermediate relay stations, all operating on different mean carrier frequencies, the system permitting two-way transmission of intelligence between any and all of the stations included therein, with the carrier Wave emanating from one of vthe terminal stations and from each intermediate relay station, hereinafter referred to as the slave stations, under the control of the carrier Wave received at each such station, and all under the control of the carrier wave originating at the remaining terminal station, hereinafter referred to as the master station.

While not limited thereto, the terminal station of the present invention is especially wel1 suited for use in a communication system which, in addition to having the foregoing characteristics, is further designed simultaneously to transmit several intelligence channels, for example, four, each of which covers a frequency band of from zero to, say, 3000 C. P. S., said individual channels being transposed by suitable channelizing equipment which forms no part of the present invention, to present a single frequency band extending from Zero to, say, 12,000 C. P. S.

In a system functioning as above described, it

is apparent that cross-talk must be held to a minimum, the overall distortion permissible being, preferably, less than 1%. In furtherance of this limitation, it is necessary that the carrier waves emanating from the various stations in the system be frequency stabilized within very close limits.

It is, therefore, the main object of the present invention to provide a terminal station for a communication system of the foregoing general description which is so designed as to satisfy the critical requirements thereof above referred to.

Now, within a terminal station of the type contemplated by the present invention it is of the utmost importance that, in so far as the transfer from one component to another of radio-frequency energy is concerned, reflection be held to a minimum. This normally presents impedance matching problems which are diicult of solution.

It is, therefore, another object of the present l invention to so design the radio-frequency portions thereof as to minimize the necessity of obtaining critical impedance matches.

As previously stated, the carrier wave emanating from each slave station in the system to which reference has been made is under the control of the carrier wave received thereby. As a result, it is necessary to eliminate in all such stations the effect thereon of reflections of the energy radiated thereby.

It is, therefore, a further object of the present invention to design a terminal station for a communication system operating as aforesaid in which the effect thereon of reflection of the energy radiated thereby is eliminated. Y

A still further object of the present invention is the provision of a terminal station which may be utilized, in a communication system of the foregoing description, either as a master station or as a slave station, its mode of functioning being controlled by a suitable switching arrangement.

These, and other objects of the present invention, which will become more apparent as the detailed description thereof progresses, are attained, briefly, in the following manner:

Throughout the remainder of this specification it will be assumed that the system comprises merely two widely separated terminal stations, with no intermediate relay stations, and with one of said terminal stations functioning as a master station and the other as a slave station. It is to be understood, however, that it is preferred that both terminal stations be so designed as to be operable, at will, as either a master or a slave station.

Each terminal station includes means for locally generating a carrier wave, preferably, in the microwave region of the electromagnetic spec- I trum, and means for frequency modulating said carrier wave with locally-generated intelligence. Normally, there is a predetermined difference between the frequencies of the carrier waves generated at each station. For example, the frequency of one of the carrier waves may be 10,000 mc. and the frequency of the other, 10,003 mc., a difference therebetween of 3 mc.

A portion of the carrier wave generated at the slave station is radiated into space, and another portion thereof is mixed with a portion of the carrier wave remotely generated at the master station, and received at said slave station, to derive therefrom a substantially constant-amplitude, intermediate-frequency wave whose frequency normally corresponds to the aforesaid frequency difference between said carrier waves. Preferably, the means for deriving said intermediate-frequency wave are such that variations in the amplitude of the locally-generated carrier wave, due, for example, to reflections of the radiated portions thereof, or to mismatches between the component parts of the station, have no effect upon the amplitude of said intermediatefrequency wave.

The intermediate-frequency wave thus derived is passed through a broad-band amplifier, whose center frequency corresponds to the frequency difference between said locally and remotelygenerated carrier waves, and thence, t a CHS:

criminator to obtain an output, having a D.C.

or unidirectional component: whose sense and; magnitude are functions, respectively, of*Y the:

sense and magnitude of any' deviation of" the frequency of said intermediate-frequency wave from the predetermined difference between said carrier waves, and having an audio-frequency component corresponding tofany'intelligence im,

pressed upon either the locally or remotely-generated carrier waves. The audio-frequency com'- ponent of the discriminator output is amplified and applied to phones, andthe D.G.' component of said output is separated.v from thev audio-frequencv component by appropriate low-pass filters, said D.-C. component sbeing applied; through a buffer amplifier, to any preferred electronic or mechanical tuning control to adjust the frequency of the localcarrier wave to compensate for any deviation from the desired mean frequency thereof'. Now, as will hereinafter be pointed out, the-remotely-generated carrier wave, emanating from thel master-'stationv and received at the slave station, is frequency-stabilized,within VVeryfclose limits, and inasmuch asthe apparatus just described assures the maintenance of a constant frequency differencevbetween the1 remotely and locally-generated carrier waves, the locally-generated carrier wave, likewise; becomes frequency stabilized.

At the master station, a portion ofthe locallygenerated' oarrierwave-is radiated; and another portion thereof is divided into two parts, one: of which is amplitude imodulated'at* a. relatively low frequency, say, 50 mc., and the other of which -is transmitted through a cavity-resonatorwhose natural resonant frequency constitutes a reference frequency corresponding to the mean frequency desired of the master-station carrier Wave. The amplitude-modulating means ,iust referred to, preferably, is such that thecarrier wave itselfV is suppressed; andfonly the resulting sidebands subsequently utilized. Byfsodoing', as will'- hereinafter become more apparent, once again'the effects of mismatches between the components of the station are eliminated.

The lengths of the paths of the portions ofthe slave-station carrier wave which are amplitude modulated and applied tothe cavity: resonator, respectively, are made -to differ by 'nA/4. where 'n' is an odd integer and t is the transmission-system wavelength corresponding to the desired frequency of said carrier wave. Hence, whenl the sidebands, produced as aforesaid'. and aportion of the carrierwave coming from the 'cavit-yrresonator are subsequently combined'in a mixer, the effect is the same-as combining a portion ofthe carrier wave which is varying in amplitude at thelmodulationfrequency with a portion of the carrier wave which is of constant amplitude and 90 out of phase with said ilrst portion, and produces a substantially constant-amplitude resultant wave, provided the frequency of said carrier wave correspondsl to the hereinbefore referred to refrence frequency, inv other words, to the natural resonant frequency of'v the cavity resonator. If; on the other hand', the`A frequency ofv said carrier wave has drifted from the frequency desired thereof, said sidebands and the carrier wave coming from the cavity resonator do not meet at the mixer in phase quadrature, and, therefore, the iii-phase components of" said. sidebands: cause the resonator-transmitted, carrier-wave portion to become substantially amplitude modulated.

The modulation envelope of the last-named resultant wave has a phase, relative to the amplitude modulation initially applied to the carrier wave, and a magnitude, which are functions, respectively, of the sense and magnitude of any deviation of the frequency of said carrier wave from thel reference frequency. Preferably, said modulation envelope is either in phase with said initially applied modulation, or it is in phase opposition. thereto.

If there has been no deviation from the reference frequency, and, as a result, a substantially constant-amplitude wave is produced at the aforesaidmixer, there-isno modulation envelope to recover, but if there has been a deviation, the modulationenvelope of the amplitude-modulated res-ultant wave produced atv said mixer, whether it is in phase with the initiallyapplied modulation, orin phase opposition thereto, is recovered, the sense andl magnitude of` the recovered envelope being functions, respectively, of the sense and'magnitude of saidl deviation.

The recovered modulation envelope isV com bined, for example, in a differential-amplitude detector, with a portionof" the-l initiallyapplied modulation to obtain an outputhaving a D. C. or unidirectional component whose sense and magnitude, likewise, are-functions, respectively, of the sense and magnitude of the deviation of the frequency of the carrier wave from itsdesired frequency, and having an audio-frequency component corresponding-- to any locally-generiated intelligence impressed upon saidl carrier wave. The output ofthe dilferentialhamplitude detector is passed through al low-pass lterto remove the audio-frequency component thereof". andthe remaining, D.C. or unidirectional' component is-applied, through a bufferamplifier, to any preferredz electronic or-- mechanical tuning control to adjust the frequency of the carrier wave to compensateA for the deviations above referred to.

In the accompanying specification there shall bedescribed; and in the annexed drawings shown', an illustrative embodiment of the terminal station ofthe present invention. It is, however, to be clearly understood that the present invention is not to be limited to the detailsv herein shown and describedl forI purposes of illustration only; inasmuch as changes therein may be made without'the, exercise ofv invention, andiwithin the true spirit andv scope ofthe claims hereto appended.

Insaid drawings,

Fig. 1 is a partial block, partial schematic diagram of 'one portion of the-terminal station ofY theA present; invention; and- Fig. 2 is a similarv view-or anothery portion of said: terminal' station.

Referring nowmore in detailf to the aforesaid illustrativeembodiment of the present invention; with particular'reference to Fig'.v 1 of the drawings, the numeral I0 designates an oscillator for locally generating, at a terminal station, a carrier wave wh, preferably, in the microwave region of the electromagnetic` spectrum. It is the mean frequency of thisy carrier wave that itis desired to stabilize, eitherI under the control' of a stabilized,I remotelygenerated carrier wave received from a master station, or by means incorporated in the local station itself.

For the present, this description will be addressed to the case where the carrier wave which is locally generated at a slave station is to be stabilized under the control of a carrier wave which is remotely generated at a master station.

The oscillator I D is adapted to be frequency modulated, by any preferred frequency -control II, in accordance with locally-generated intelligence we which is applied to said frequency control through a microphone I2 and an audiofrequency transformer I3. The output of the oscillator Ill, consisting of the carrier wave we and any intelligence we, impressed thereon, is applied to one of the side bran-ches I4 of a wave-guide assembly I5, known as a magic T. Such a Wave-guide assembly comprises, in addition to the side branch I4, another side branch it, eX- tending in the same direction as the side branch I il, a so-called E arm I'I extending outwardly from said side branches lll and i6, at right angles thereto, and a so-called H arm I9 extending outwardly from said side branches le and i6, mutually perpendicular to said side branches and said E arm, said side branches, said E arm, and said H arm all extending from a common junction I9. As shown in the drawings, the L arm I8 recedes from the observer for a short distance from the common junction i9, then bends downwardly, and is then twisted through an angle of 90.

As the energy travelling along the side branch Ill from the oscillator IE! reaches the common junction I9, it splits, part travelling along the H arm I'8 to a matched load 2U which absorbs the same, part leaking across the T and travelling along the side branch I and part travelling along the E arm ll to an electromagnetic horn 2l adapted to directionally radiate the same. The horn 2| is also receptive, in the opposite direction, of a remotely-generated carrier Wave, of a frequency wh, which may be frequency modulated with intelligence w'a, and which emanates from another terminal station, inthe case under consideration, a master station, the master and slave station carrier waves normally differing, as hereinbefore stated, by a predetermined frequency wd.

Now in certain instances, depending upon the location of the slave station, the horn 2l is additionally receptive of reflections of the energy radiated thereby, that is reflections of the carrier wave wh. These reections travel along the E arm I l, and combine with the energy travelling along the side branch I6 to cause variations in the amplitude of the latter, and upon combination, as indicated in earlier portions of this specication, of the locally and remotely-generated carrier waves, to obtain an intermediatefrequency Wave corresponding in frequency to the dierence wd between the frequencies of said carrier waves, said intermediate-frequency wave becomes undesirably amplitude modulated. In order to eliminate this undesirable amplitude modulation. the intermediate-frequency wave is derived in the following manner.

The side branch Il of the magic T l5 is extended, at 22, to form, in effect, the E arm of another magic T 23, which also includes two oppositely-extending side branches 2d and 25, and an H arm 26, all projecting from a common junction 2. The side branches 2d and 25 are terminated in oppositely-disposed crystals 28 and 29, between one side of ea-ch of which and the magic T itself, capacitances 3B and 3| exist, said magic T being grounded, and said crystals being connected, in series with a source 32 of direct current and a resistor 33, to provide parallel outputs to ground through a capacitor 34 and the primary winding 35 of a radio-frequency transformer 36. The source 32 of direct current is for the purpose of operating the crystals 28 and 29 at a favorable point along their characteristic curve, and the series circuit is for the purpose of passing equal currents through said crystals to assure their similar action even though they both be mismatched to the Wave-guide system. The H arm 2G is connected, through a Wave guide 3l and a unidirectional 4coupler 33, to the side branch Id of the magic T I5, whereby said I-I arm is receptive only of energy `un coming directly from the oscillator Il), and cannot receive any energy w'h travelling from the common junction I9 toward said oscillator I0.

For an understanding of the operation of the magic T 23, assume that the carrier waves wa and wh are both travelling from the junction I9, along the side branches I6 and 22, toward the junction 2l, with their electric vectors in the same direction, arbitrarily, upwardly. As they reach the junction 2l, each splits into two constituent Waves, the Waves travelling along the side branch 2li, toward the crystal 28, having their electric vectors pointing in the same direction, and the waves travelling along the side branch 25, toward the crystal 29, having their electric vectors, likewise, in the same direction, which is the opposite direction to that of the vectors at the crystal 28. At the crystal 28, a beat-frequency wave, whose frequency corresponds to the frequency difference between the carrier waves, and whose phase may be considered positive because its component waves are in phase, is produced; and at the crystal 29, another beat-frequency wave, having the same frequency as said rst-named beat-frequency Wave, and having a phase which may also be considered positive because its component waves are also in phase, is produced. However, as to each other, the two beat-frequency waves thus obtained are out of phase and set up equal voltages of opposite polarity across the crystals. Inasmuch as parallel outputs are taken from these crystals, cancellation occurs and no output appears across the primary winding 35 of the transformer 39. Thus, the effects of variations in the amplitude of the locally-generated carrier Wave wh, due, for example, to reections of said carrier wave received by the horn 2l, are eliminated from the system.

When the remotely-generated carrier wave wh, received at the slave station and travelling along the E arm 22 toward the junction 2l, is mixed with the locally-generated carrier wave wh, coming directly from the oscillator I through the unidirectional coupler 38, and travelling along the H arm toward the junction 2l, a different result is obtained. Assume, as before, that the electric vector of the remotely generated carrier wave wh, travelling along the E arm 22, is pointing upwardly, and further assume that the electric vector of the locally generated carrier wave wh, travelling along the H arm 26, is pointing to the left at the junction 2l. When the former reaches the junction 2l, it splits into two constituent waves, one, having its electric vector pointing to the right, travelling toward the crystal 28, and the other, having its electric vector pointing to the left, travelling toward the crystal 29. When the latter reaches the junction 2l, it, too, splits into two constituent-waves, one travelling toward the crystal 28 and the Aother travelling toward the crystal 29, with both having their electric vectors pointing in the same direction, namely, to the left. Thus, therel arrive at the crystal 28 two constituent carrier waves which are out of phase, and there arrive atvthe crystal 29 two vconstituent carrier waves which are in phase. Therefore, there is producedl at the crystal 28, a beat-frequency wave whose frequency corresponds to the difference 'be-tween the frequencies of the carrier waves, andi whose phase may be considered negative because-itscomponent waves are out of phase; while at the crystal 29, there is produced a 'beat-frequency wave having the same frequency as said'ilrst-named beat-frequency wave, and having a phase which may be considered positive because its component waves are in phase. However, as to each other, the two beat-frequency waves thus obtained are in phase, and set up equal voltages of.' like polarity across the crystals. Inasmuch -as parallel outputs are taken from these crystals, addition occurs, and a constant-amplitude intermediate-frequency wave appears across the primary winding 35 of the transformer 3E.

Through a secondary winding 3S of the transformer 35. this intermediate-frequency lwave is applied to a broad-band: amplifier l5 whose Icenter frequency Acorrespondsvto the normal difference wd between the frequencies of the locally and remotely-generated carrier waves. The output of the-amplifier 40, consisting of the difference frequency ed and any audio-frequency modulation onor oa originating, respectively, at the slave s and master stations, is applied to a frequency discriminator 4l whose center frequency corre- -spon'ds to said normal' difference frequency wd between the .carrier waves. The discriminator extracts` any intelligence impressed upon the intermediate-f-requency input applied thereto, which intelligence is then` conveyed, through an audio-frequency ampli-fier l2 and an audio-frequency transformer 43, to phones 44.

As long as the inea-n frequency of the intermediate-frequency wave derived from the heterodyning of the remotely and locally-generated carrier waves correspondsl tothe predetermined difference wd therebetween, the only output from the discriminator M1 is an audio-frequency output corresponding to the intelligence wa or we; but, should the oscillator L' drift, resulting in a deviation of the frequency rof the intermediatefrequency wave from the difference od between the carrier-wave frequencies, the discriminator 4`|= will have an output including, in addition to that resulting from the intelligence applied thereto', a D.C. or unidirectionalI component whose sense and magnitude will depend, respectively, on. the sense and magnitude of any such deviat-ion. Therefore, the output ofthe discriminator is applied to a `low-pass filter 55 to recover said D'.-C. component, which may then be applied, through a switch 45, to a buffer amplifier dfi, and thence, to the frequency control lll to adjust the oscillator I0 and compensate for any frequency deviation from the mean frequency desired of said' oscillator.

By means of the apparatus thus far described, the frequency of a locally-generated, slave-station carrier wave maybe stabilized under the control of a, remotely-generated, master-station carrier Wave.

Inl order to enable the terminal station of the present invention to. functionuas a master station as well as a slave-station, another portion of the locally-generated carrier Wavey from the oscillator is conveyed, through a' unidirectional coupler 4B, to a wave-guide 4&9-, and thence, (see Y, Y, Figs. l and 2) to the E arm v5l) of a magic T 5l The latter includes, in addition tothe E arm 55, side branches 52 and v56S vand an H arm '54', all exteiding from a common junction 55.

The energy travelling along the E arm 5B splits at the junction 55, part travelling along the side branch 52 part travelling along the side branch 53, and part travelling along the H arm '54 which is terminated in a matched load 56.

The side branch 52' communicates with the E arm 51 of another magic T 58 including side branches 59 and 60 and an H arm 5l, all extending from a common junction 52, and the side `branch 53 communicates, through a wave guide 63 incorporating a line stretcher 65 for adjusting the electrical length thereof, with a cavity resonator 65 the natural resonant frequency wh ofwhich constitutes the reference frequency with respect to which it is desired to maintain the frequency of the oscillator l5 in a fixed relationship;

The side branches 59 and 6i! of the magic T 58 terminate in oppositely-disposed crystals S6 and El, between one side of each of which and the magic T itself, capacitances 53 and 6'9 exist, the wave-guide System, as previously indicated, being grounded, and said crystals being connected, in series with a source l5 of direct current and a resistor ll, to provide parallel inputs to ground through a capacitor l2 and the secondary Winding 'i3 of a radio-frequency transformer 'M The primary winding T5 of the transformer lll is connected to an oscillator I6 adapted to generate relatively low, radio-frequency energy w.- for amplitude modulating the locally-generated carrier wave wh. The lfrequency of the modulating energy may be, for example, 5G mc.

The magic T 58 functions as a balanced modulator. The carrier wave wh travelling along the E arm 5l toward the junction 6.2, with or without locally-generated intelligence wa, splits at said junction, part travelling along the side branch 59 toward the crystal 66, and part travelling along the side branch B toward the crystal El. The oppositely-travelling parts of the carrier waves incident upon said crystals are,v it will be noted, out of phase., and 4,each become amplitude modulated by the Venergy injected into said crystals in parallel from thev oscillator l5. Thus, sidebands, having frequencies equal to the sum of and difference between the frequencies of the carrier and modulation Waves, are produced at the crystalsr 65 andy 6,1., which sidebands, together with any carrier, wave .components resulting from mismatching .of the crystals to the wave-guide system, travel back along the side branches 59 and 60 toward the junction 62.. The sidebands, upon reaching .said junction, travel along the H arm 6l of the magic T 58, but the reiiected carrier-wave components, being in phase opposition, are cancelled along said H arm'l, so that the final output of the lmagic T 58 is a. carrier-suppressed,v a-mplitude-modulated wave.

The sidebands comprising said last-named wave are conveyed, through a wave guide 1l, to the E arm 'i8 of still another magic T 'I9 which, like those previously described, includes side branches 89 and 8| and an H arm 82, al1 extending from a common junction 83. The side branches 8.0 and 8l areterminated in crystals 84. and 85, between one. side off each of which.

and the magic T itself, capactances 86 and 81 exist, said magic T being grounded, and said crystals being connected, in series with a source 88 of direct current and a resistor 89, to provide parallel outputs to ground through a capacitor 98 and the primary winding 9| of a radio-frequency transformer 92. The H arm 82 of the magic T 'I8 is connected, through a wave guide 93, to the output side of the cavity resonator 85.

By adjusting the line stretcher 68, the electrical lengths of the paths taken by the energy travelling from the junction 55 of the magic T to the junction 83 of the magic T le, through the balanced modulator magic T 58 on the one hand, and through the cavity resonator 85 on the other, are made to differ by 11A/4, where n is an odd integer and l is the waveeguide wavelength corresponding to the natural resonant frequency of the cavity resonator 85, in other words, to the mean frequency desired of the locally-generated carrier wave coming from the oscillator I0.

Now at the junction 88 of the magic T 'I9 which, as will now be described, functions as a balanced demodulator and a buffer isolating the balanced modulator magic T 58 from the cavity resonator 85, the sideband Waves travelling along the E arm i8 split, part travelling along the side branch 8B toward the crystal 8d, and part travelling along the side branch 8| toward the crystal 85. Also at said junction 83, the carrier Wave wh, travelling along the H arm 82 from the cavity resonator 65, splits, part travelling along the side branch 88 toward the crystal 84, and part travelling along the side branch 8i toward the crystal 85.

At each of the crystals 84 and 85, the upper sideband may be considered as a vector rotating in a counter-clockwise direction at the frequency of the carrier wave wh plus that of the modulation wave w1, and the lower sideband may be considered as a vector rotating in the same direction at a frequency of the carrier wave minus that of the modulation Wave. These two vectors, rotating at different frequencies, periodically become in phase, and so, they may be considered as oppositely-rotating vectors combining to produce a non-rotating vector constituting a resultant sideband. The latter is, actually, the carrier wave varying in amplitude at the modulation frequency, and when combined, in each of the crystals 813 and 85, with carrier-wave portions coming from the cavity resonator 65, with which they are 90 out of phase, the amplitudes of said carrier-wave portions remain substantially unaffected. Therefore, no modulation envelopes are recovered at the crystals 84 and 85.

However, should the oscillator I0 drift, so that the frequency of the locally-generated carrier wave differs from the reference frequency, that is, from the natural resonant frequency of the cavity resonator B5, the latter causes the phase of the carrier wave transmitted therethrough to the balanced demodulator magic T I9 to shift in a direction and to an extent depending, respectively, on the direction and extent of the frequency drift. Hence, at the crystals 84 and 85, the sideband resultant waves and the carrier-wave portions coming from the cavity resonator are no longer in phase quadrature, and as a result, inphase portions of said sideband resultant Waves cause said carrier-wave portions to become amplitude modulated.

If the frequency of the carrier wave generated by the oscillator I0 drifts above the reference fre- 10 quency, the carrier-Wave portions coming from the cavity resonator 65 acquire 1 amplitude modulation7 and if said frequency drifts below the reference frequency said carrier-wave portions acquire amplitude modulation.

In any event, the modulation envelopes thus produced are recovered across the crystals 84 and 85, and add to each other to appear across the primary winding 9i of the transformer 92.

It now remains to extract from the output of the balanced demodulator magic T 19, a D.C. or unidirectional component of proper sense and magnitude to adjust the frequency control II and compensate for the deviations of the oscillator I0. For this purpose, the secondary winding 94 of the transformer 8L? is connected to an amplifier 95 which is tuned to the frequency w1 of the modulation oscillator i6. The output 0f the amplifier 95 is applied to a differential-amplitude detector 96 to which there is also applied a portion of the output of the modulation oscillator 16. Such a differentiahamplitude detector may, for example, be similar to the one disclosed in the copending application of George G, Bruclr` and Philip E. Volz, entitled Frequency-stabilizing systems, Serial No. 647,008, filed February l2, 1946.

The differential-amplitude detector 88 is so designed that, in the case where the oscillator I0 has not drifted at all, and as a result, no modulation envelope appears across the transformer 92, said detector has a balanced output which cancels itself. Said detector is so further designed that, in the case where the oscillator I8 has drifted, and, as a result, a modulation envelope does appear across the transformer 82, said detector produces a unidirectional output whose sense and magnitude are functions, respectively, of the sense and magnitude of said drift.

In order to remove from the output of the differential-amplitude detector any intelligence we which may have been initially impressed upon the carrier wave wh, said output is applied to a low-pass lter 9'I, thereby obtaining a final D.C. output. This output is then applied, through a switch 98, (see X, X-X, X, Figs. 1 and 2) to the buffer amplier 51, and thence, to the tuning control I I to compensate for any drift in the oscillator I8.

Obviously, when the terminal station of the present invention is functioning as a master station, the switch 4B is open and the switch 98 is closed, and when the station is functioning as a slave station, the switch 48 is closed and the switch 9S is open.

This completes the description of the aforesaid illustrative embodiment of the present invention, including the mode of operation thereof as either a master or a slave station in a communication system having the characteristics set forth in the opening paragraphs of this specification.

It will be noted from all of the foregoing that the present invention enables the stabilization of a communication system carrier wave within very close limits. Further, it enables the designing of a communication system terminal station in which the necessity of critical impedance matches between the component parts is minimized. Still further, it avoids the deleterious effects on the entire system of reflections of the energy radiated by the various stations included therein. Finally, it provides a terminal station which may be made to function either as a master station controlling the various carrier waves employed in the system, or as a slave station whose carrier wave is controllable under the iniiuence of a remotely- J .i generated carrier wave emanating from another .terrninalstatiom itself functioning asa master station- Other objects and t.advantages of the present invention will readily oecur .to those skilled in Ltheartto which the f samerelates.

vWhat is. claimed is:

1. In a terminal station for la communication system: means for locally generating a carrier wave; means in common, yfor radiating said locally generated carrier wave receiving any re- Aiiections thereof, and receiving a remotely generated carrier wave; said locally and remotely generatedcarrier waves tn or-mally having a predetermined frequeney :difference therebetween; means, Areceptive of `p ontitnis zof said locally vEenerated carrier wave, tany lreflections thereof, and said remotelygeneratedcarrier wave from said last-named-meanafor dividing thelsame into two p airs .of constituent waves, the waves of each H pair being of YVlike phaseand .the pairs being of opposite. phase, andreceptiveof `.a portion of said remotelygenerated,carrier wave from said lastnamed fmeans andaportion of .said locally generatedcarrier wave` directly Vfrom said means :for generating fthe same, for dividing the same into ,two lfadditional 4pairs Iof :constituent waves, the yvavesnfone..of?said additional -pairs being of like phasetandrthewaves ofthe other additional vpair being of ,opposite phase; means for mixing -the constituentfwaves of each .of said 4rst-named pairs to -derive theref :two beat-frequency 4waves :of equalamplitude and like -phasefand for mixing the constituent iwaves -of each of said seczondmamed pairs to derive therefrom two beatfrequency wax/es r of equal amplitude and QDDoSite phase; AVmeans V1for vso v'combining said rst-named beatfrequency *.Waves as -to `produce a zeroutput,.andforso combining said secondnamed fbeat frequency waves as to produce a single `substantially constant-,amplitude intermediate-frequency -wave whose frequency `normall-v corresponds .tosaid -nredetermined differ.- ence .between the ,frequencies of said ,carrier waves and means, .receptiveofsaid intermediatefrequency'waxie, .for/deriving therefrom `an output for SO controlling the `Eire q uency .of said :locally generated carrier wave as to maintain 1,Substantiaily ,constant saidfpredetermined difference lbetween the fredueueles lef vsaid eat-tier waves.

,2. `Zina communicationfsystem: a master station; .a Slave `Statlineflllflllely located With freslpect to said 7master station;v .means -atleach 4of said stations for locally generatinlgaicarrier wave; the carrier .Waves Iso ,generated normally `havfi ng a, ,predeterm ined frequency .difference therebetween; means at saidlnaster station for -auto- Illa-dually stabilizing the ,frequency of the .carrier wave generated ,thereat .independently of yany drift in the Efrequency. of. the carrier wave generated at saidslave station; means Kat saidslaye station, receptiveof portionsof bothisaid carrier waves, for deriving from ,one ,two constituent Waveeof equal amplitude andfopposite phase'and fram the other., twdednstituent waves .of equal amplitude .and like phase: means .for .deriving from all Vof said oonstitllillt waves an 4 intermedi-- ateffrequency wave `llt/ hose frequency normally corresponds to said predetermined frequency dif.- ference .between .said carrier waves; means, recptive of said int ermediate-gfiefluerlct7 Wave, for deriving ,therefrom a .unidirectional .output .whose sense and magnitude ,are fnnctions, respectively, of thesense and magnitude .of any deviation .of

the frequency of ,said ,intermediaterfrequency wave vfrom .said predetermined ,frequency differ, ence between said carrier waves; and means, receptive ,of said unidirectional output .for 4automatically/so controlling the frequencyof the carrierwave generateclfat said slave stationas automatically to maintain substantially-constant said predetermined frequency difference between said carrier waves, .thereby automatically stabilizing the frequency of .the carrier wave generated at said slave station.

3. Ina communication system: a master station; a slavesltton remotely located with re.- spect to said master station; means .at each of ,said stations forlocally generating a vcarrier wave; the carrier wavesso `generated normally having a predetermined ,frequency difference therebetween; ,means at said master station for automatically stabilizingthe frequency of the carrier wave generated thereat independently of any drift in .the frequency of the carrier wave generated at said slaverstation; ,means at said slave station, receptive of a portion of one of said carrierwaves, Afor dividing the 4same into two constituent waves of .equal amplitude and ropposite phase; v means at said slave station, receptive ofa portion of.. tl 1 e remaining carrier Wave, fordividing the same Yinto twoconstituent waves of .equal amplitude and like phase; means for mixing said first-named constituent waves, respectively, with said second-named constituent Waves to derive therefrom two beat-frequency waves lof equal `amplitude and opposite phase; means for so combining saidbeat-frequency waves as to produce ,a single intermediate-frequency Wave whose frequency normally corresponds to Said predetermined frequency difference between Vsaid carrier waves; means, receptive of said intermediate-.irequency wave, for deriving therefrom a unidirectional output whose sense and magnitude are functions, respectively, of `the sense and magnitude of any deviation of the frequenoyofsaid intermediateffrequency wave from said predetermined 'frequency difference between said carrier waves; and means, receptive of said unidirectional output, for automatically so controlling. the frequency of the carrier wave generated vat said slave station as automatically to maintain substantially constant said predetermined Ffrequency difference 'between said carrier Waves, thereby automatically,stabilizing the frequencypf i',heearrier wave generated at said slave station.

4. In a Icommunication system: a master station; a slave station remotely located with re- Spett .t0 .Said master Station; means at each of said stationsforlooally generating .a carrier wave; the carrier waves sogeneratedhavinga predetermined .frequency difference therebetween: means at ,said master statiomreceptive of a portion o f the.. carr ier wave generated thereat, for derivine therefrom .a earnerfsuppressed, amplitudemodulated wave; means at said master station, receptive of.Y andther ,portion .of Ythe carrier wave generated ,thereat forderiving ,therefrom a wave whoseV phase has ,been ,shifted .through 4am angle of A90?; means atsad masterstation, responsivev 13 lated wave the sense and magnitude of Whose modulation envelope are functions, respectively, of the sense and magnitude of said frequency drift; means at said master station, receptive of said resultant Wave, for deriving therefrom a unidirectional output whose sense and magnitude likewise are functions, respectively, of the sense and magnitude of said frequency drift; means at said master station, receptive of said unidirectional output, for so controlling the frequency of the carrier wave generated thereat as to compensate for said frequency drift and thereby stabilize said carrier Wave; and means at said slave station, receptive of a portion of the carrier Wave generated at said master station and a portion of the carrier Wave generated at said slave station, for maintaining substantially constant said predetermined frequency difference therebetween, thereby stabilizing the frequency of the carrier Wave generated at said slave station.

GEORGE G. BRUCK. PHILIP E. VOLZ. PAUL J. PONTECORVO. MALCOLM C. VOSBURGH.

REFERENCES CITED The folloiwing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,343,307 Carson June 15, 1920 1,626,724 Demarest May 3, 1927 1,921,168 Royden Aug. 8, 1933 2,317,547 McRae Apr. 27, 1943 2,333,719 Herold Nov. 9, 1943 2,379,395 Ziegler June 26, 1945 2,408,791 Magnuski Oct. 8, 1946 2,408,826 Vogel Oct. 8, 1946 

